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Abstract

We study disorder-induced propagation losses of guided modes in photonic crystal slabs with line-defects. These losses are treated within a theoretical model of size disorder for the etched holes in the otherwise periodic photonic lattice. Comparisons are provided with state-of-the-art experimental data, both in membrane and Silicon-on-Insulator (SOI) structures, in which propagation losses are mainly attributed to fabrication imperfections. The dependence of the losses on the photon group velocity and the useful bandwidth for low-loss propagation are analyzed and discussed for membrane and asymmetric as well as symmetric SOI systems. New designs for further improving device performances are proposed, which employ waveguides with varying channel widths. It is shown that losses in photonic crystal waveguides could be reduced by almost an order of magnitude with respect to latest experimental results. Propagation losses lower than 0.1 dB/mm are predicted for suitably designed structures, by assuming state-of-the-art fabrication accuracy.

Figures (6)

Schematic pictures of systems under study in the present work. (a) Line-defect along the Γ-K direction for the unperturbed and disordered triangular lattices in the (x,y) plane; the effect of a random variation of holes radii has been exaggerated. Light gray holes represent the fundamental cell to be repeated with supercell periodicity in a square lattice. The main symmetry directions of the triangular lattice, Γ-K and Γ-M, are also defined. (b) Vertical guiding structures considered in this work: (from left to right) the air-clad, the asymmetric SiO2-clad, and the symmetric SiO2-clad photonic crystal waveguides.

Calculated photonic dispersion and propagation losses of guided modes in reduced-width PhC waveguides realized in a SOI structure, as schematically represented in Fig. 1(b) (picture in the middle), for different widths of the waveguide channel. Parameters are: d/a=0.5, r/a=0.26, a=400 nm. The highlighted spectral region on the right refers to the σ
kz
=-1 defect mode with the largest low-loss propagation bandwidth (W0.65 waveguide).